This invention relates to cooking vessels of stainless steel with good boiling, baking and frying properties, irrespective to the heat source employed.
The use of stainless steel for making cookware has been known for many years, this metal being in many ways suitable for preparing food even though its cooking properties are not entirely good due to its relatively poor thermal conductivity. The various types of stainless steel have properties which, without modification, make them unsuitable for cookware which is to be used also for induction heating. Various attempts have been made to overcome these difficulties.
Austenitic stainless steel has very low magnetic permeability, and is thus not a suitable material for cookware for heating by all types of heat sources. Very good cooking properties have however been imparted to such cookware by strengthening the base with a plate of a metal with good thermal conductivity, for example aluminum or copper, thus making the cookware suitable for heating on an ordinary hotplate or by gas.
The bond between austenitic stainless steel and aluminium or copper has proved to be very strong, so that the extra base does not fall off, not even after prolonged use. However, attempts to heat up such a vessel on an induction plate are not successful because the metals of the vessel cannot, in practice, be magnetized.
Chromium steel, with its good magnetic permeability, has also been tried as a material for cookware. As with cookware of austenitic steel, it has been desirable to improve the cooking properties by brazing a copper plate onto the base, and a protective plate of chromium steel outside the copper plate. A saucepan or other type of cooking vessel of this type may be efficiently heated by an induction plate, but the base is not durable. The combination of chromium steel and copper suffers from too large a difference in the coefficients of thermal expansion so that the base becomes loose after only a short period of use.
Also, martensitic stainless steel has very good magnetic permeability. One proposal employs cookware of copper or aluminium clad on the outside and the inside with this stainless steel. However, the combination of martensitic stainless steel and copper suffers from the same large difference in the coefficients of thermal expansion as does the chromium steel-copper combination, and thus does not last. An even larger difference is shown in the combination of chromium steel and aluminum, and therefore this combination also is non-durable.
It thus has been proved in practice that the two above-mentioned types of stainless steel which have good magnetic permeability, i.e. chromium and martensitic, cannot, with conventional technology, be affixed firmly enough to aluminum or copper to be able to withstand the type of heating which is produced by high frequency electrical induction heating. The question of extending the range of use of stainless steel saucepans or other types of cooking vessels to include heating on induction plates has thus not been satisfactorily solved.